1. Field of the Invention
The invention relates to branched organosiloxane (co)polymers whose branching is composed of at least trivalent organic building blocks and whose ends are formed by organic groups attached via Sixe2x80x94C bonds, to a process for their preparation, and to their use as antimisting additives for reducing the formation of aerosols during manufacture of crosslinkable silicone coating compositions.
2. Background Art
Branched organopolysiloxanes whose branching units correspond to customary monoalkylsiloxy units are described in U.S. Pat. No. 5,670,686. Starting from prefabricated siloxanes containing three or four hydrodimethylsiloxy units, a single-stage hydrosilylation process produces branched siloxanes having different end groups. Polysiloxanes with higher degrees of branching are not described; all branching sites correspond to those derived from conventional silicone chemistry. The xe2x80x9chydroxe2x80x9d precursors, such as tris-[dimethylsiloxy]allylsilane, are prepared in a hydrolytic process and purified by distillation.
Siloxanes of higher molecular weight can be obtained only by equilibration. In such equilibration reactions, it is necessary to use acidic catalysis, which is not fully satisfactory with siloxanes containing alkylsiloxy units. The need for acidic catalysis is dictated by the fact that, in the case of substances containing hydrosiloxy groups, basic catalysis leads to chemical decomposition.
Products with higher degrees of branching can be obtained according to Polymer Preprints 34(1), 77 (1993) from an AB3 monomer by using tris[dimethylsiloxy]allylsilane as monomer and by noble metal catalysis obtaining a highly branched carbosiloxane intermediate containing hydrodimethylsiloxy end groups which is then reacted with xcex1-unsaturated organic compounds. This method, involving polyaddition, requires the preparation of an extremely high-energy monomer whose handling represents a considerable safety risk. The branching is based exclusively on customary T units (SiO3/2 units). Siloxanes containing dialkylsiloxy units are not described and are also virtually unobtainable by known methods. A similar methodology is described in PMSE Prepr. 80, 139 (1999): H dendrimers are obtained from bis[dimethylsiloxy]methylvinylsilane by polyaddition and are subsequently reacted with xcex1-unsaturated organic compounds.
DE-A 195 22 144 describes branched siloxane copolymers which are obtained from compounds containing at least 3 double bonds reactive with respect to hydrosilylation, reacting these compounds with xcex1,xcfx89-dihydrosiloxanes, and subsequently reacting the intermediates generated in the 1st stage with xcex1,xcfx89-dienes such as 1,5-hexadiene. For the preparation of branched siloxanes, however, this process is unsuitable, giving only insoluble gels, particularly if, for economic reasons and in consideration of the desired product purity, the use of relatively large excesses of xcex1,xcfx89-dienes is avoided. Moreover, branched products which are inert toward hydrosiloxanes cannot be prepared in this way.
Regular dendrimers are obtainable in accordance with Polymer Preprints 39 (1), 581 (1998) if tetraallylsilane is reacted repeatedly with methyldichlorosilane and subsequently with an allyl-Grignard solution. The dendrimers are allyl-functional and can be hydrosilylated in a subsequent reaction.
Conventional dendrimers are obtained in accordance with U.S. Pat. No. 6,184,407 by reacting siloxanes containing two or more SiH groups with vinylalkoxy silanes and exchanging the now polymer-bound alkoxy groups hydrolytically for SiH-containing siloxy groups to give dendrimers which are able in turn to react with vinyl-functional components. The preparation process is complicated and expensive.
A trend within the silicone coating industry is to increase machine speed in order to raise productivity. Where silicone coatings are applied to a substrate at relatively high speeds, for example speeds of more than 300 m/min, fine spray mists of the silicone coating system may be formed. These aerosols are formed at the silicone applicator unit. The formation of this spray mist proves to be a serious problem in the context of further increases in the coating speed, and can be reduced by adding what are known as antimisting additives to the silicone coating system.
EP-A 716 115 (Dow Corning Corp.) describes antimisting additives obtained by reacting an organosilicon compound, a compound containing oxyalkylene groups, and a catalyst. The addition of these oxyalkylene-functional reaction products to crosslinkable silicone coating systems reduces the formation of aerosols in rapid coating processes. The antimisting additives, which contain polyglycol groups, have the disadvantage, however, that they are poorly miscible with the crosslinkable silicone coating compositions.
WO 01/98420 (Dow Corning Corp.) discloses a liquid silicone antimisting composition obtained by reacting an organohydropolysiloxane containing at least 2 Sixe2x80x94H groups with an organoalkenylsiloxane containing at least 3 alkenyl groups in the presence of a platinum catalyst, and optionally, an inhibitor, in a Cxe2x95x90C/SiH ratioxe2x89xa74.6. An extremely large excess of organoalkenylsiloxane (Cxe2x95x90C) is needed in order to prevent gelling. This excess influences the release properties of the base system, i.e., the crosslinkable silicone coating composition. It is generally necessary rather than optional to include an inhibitor in order to prevent gelling.
It is an object of the present invention to provide moderately to highly branched organosiloxane (co)polymers which bear Sixe2x80x94C-bonded organic groups at branch termini, and which can be prepared simply without complicated multistage processes, while at the same time avoiding the use of starting materials which are expensive and/or hazardous to handle. Thus, one object of the invention is to avoid use of xe2x80x9cmonomericxe2x80x9d building blocks such as tris[dimethylsiloxy]allylsilane which are not only expensive, but in addition do not allow any variability in chain length between the branching sites, and which pose a high safety risk when handling. A further object of the invention is to provide antimisting additives for crosslinkable silicone coating compositions which reduce the formation of aerosols in rapid coating processes, which are readily miscible with the silicone coating compositions, and which do not impair the performance of silicone coating compositions.